Table of Contents
pnmhisteq - histogram equalise a portable anymap
pnmhisteq
[-gray] [-rmap pgmfile] [-wmap pgmfile] [-verbose] [pnmfile]
pnmhisteq
increases the contrast of a portable graymap or pixmap through the technique
of histogram equalisation[1]. A histogram of the luminance of pixels in
the map is computed, from which a transfer function is calculated which
spreads out intensity levels around histogram peaks and compresses them
at troughs. This has the effect of using the available levels of intensity
more efficiently and thereby increases the detail visible in the image.
Mathematically, if N[i] is the number of pixels of luminosity i in the
image and T is the total number of pixels, luminosity j is replaced by:
j
---
\
> N[i] / T
/
---
i=0
If you're processing a related set of images, for example frames of an
animation, it's generally best to apply the same intensity map to every
frame, since otherwise you'll get distracting frame-to-frame changes in the
brightness of objects. pnmhisteq's -wmap option allows you to save, as a portable
graymap, the luminosity map computed from an image (usually a composite
of the images you intend to process created with pnmcat). Then, you can
subsequently process each of the individual images using the luminosity
map saved in the file, supplied with the -rmap option.
- -gray
- When processing
a pixmap, only gray pixels (those with identical red, green, and blue values)
are included in the histogram and modified in the output image. This is
a special purpose option intended for images where the actual data are
gray scale, with colour annotations you don't want modified. Weather satellite
images that show continent outlines in colour are best processed using
this option. The option has no effect when the input is a graymap.
- -rmap
mapfile
- Process the image using the luminosity map specified by the portable
graymap mapfile. The graymap, usually created by an earlier run of pnmhisteq
with the -wmap option, contains a single row with number of columns equal
to the maxval (greatest intensity) of the image. Each pixel in the image
is transformed by looking up its luminosity in the corresponding column
in the map file and changing it to the value given by that column.
- -wmap
mapfile
- Creates a portable graymap, mapfile, containing the luminosity
map computed from the histogram of the input image. This map file can be
read on subsequent runs of pnmhisteq with the -rmap option, allowing a group
of images to be processed with an identical map.
- -verbose
- Prints the histogram
and luminosity map on standard error.
All flags can be abbreviated to their
shortest unique prefix.
Histogram equalisation is effective for increasing
the visible detail in scientific imagery and in some continuous-tone pictures.
It is often too drastic, however, for scanned halftone images, where it
does an excellent job of making halftone artifacts apparent. You might
want to experiment with pgnnorm, ppmnorm, and pnmgamma for more subtle
contrast enhancement.
The luminosity map file supplied by the -rmap option
must have the same maxval as the input image. This is always the case when
the map file was created by the -wmap option of pnmhisteq. If this restriction
causes a problem, simply adjust the maxval of the map with pnmdepth to
agree with the input image.
If the input is a PBM file (on which histogram
equalisation is an identity operation), the only effect of passing the
file through pnmhisteq will be the passage of time.
pgmnorm(1)
,
pnm(5)
, pnmcat(1)
, pnmdepth(1)
, pnmgamma(1)
, pnmnorm(1)
- Russ, John C. The
Image Processing Handbook. Boca Raton: CRC Press, 1992. Pages 105-110.
Copyright
(C) 1995 by John Walker (kelvin@fourmilab.ch).WWW home page: http://www.fourmilab.ch/
Permission to use, copy, modify, and distribute this software and its
documentation for any purpose and without fee is hereby granted, without
any conditions or restrictions. This software is provided ``as is'' without
express or implied warranty.
Table of Contents